The preparation of 2-(methylthio)barbituric acid by methylation of the sodium salt of 2-thiobarbituric acid is described in the prior art. The methylating reagents used in the prior art are dimethyl sulfate (German Offenlegungsschrift 2,412,854, Example 12; and German Offenlegungsschrift 2,523,324, Example c), or methyl iodide (U.S. Patent No. 4,199,583). Despite the use of these particularly reactive alkylating agents, the yields of 2-(methylthio)barbituric acid remained unsatisfactory (33% in German Offenlegungsschrift 2,523,324; and 89.3% in U.S. Pat. No. 4,199,583), and the product was of insufficient purity. Moreover, dimethyl sulfate and methyl iodide are toxic substances and therefore require extensive safety precautions.